1. Field of the Invention
The present invention relates to a recording method for recording optical data to a writable data recording medium, and to the structure of a data recording medium used by this method.
2. Description of the Related Art
Devices for recording and reproducing optical data, particularly digital data, to data recording media have been the subject of much development due to the ability of such devices to store large volumes of data using media of a given physical size.
The phase change optical disk is one type of recordable optical data recording medium. To record to a phase change optical disk, the beam from a semiconductor laser is focused on the rotating disk to heat and melt, that is, change the phase of, a recording film. The temperature of the recording film and the rate at which the film cools vary, for example, according to the intensity of the optical beam.
When the intensity of the optical beam is high, the film cools rapidly from a high temperature state, and the recording film is changed to an amorphous phase. When the optical beam is relatively weak, the recording film cools gradually from a medium-high temperature state, and the recording film thus crystallizes. The resulting amorphous areas of the recording film are normally referred to as a "mark" the crystallized part between consecutive marks is normally referred to as a "space." These marks and spaces can be used to record two-value data, that is, 0s and 1s.
It is also to be noted that laser power, when the optical beam intensity is high, is referred to as "peak power," and laser power, when the optical beam intensity is low, is referred to as "bias power."
When reproducing data, a low power optical beam, that is, a light beam not strong enough to produce a phase change in the recording film, is emitted to the disk, and the light reflected back from the disk is then detected. In general, the reflectance of the amorphous phase marks is low, and the reflectance of crystal phase spaces is high. A reproduction signal can therefore be obtained by detecting the difference in the amount of light reflected from the marks and spaces.
Mark position recording (or PPM recording) whereby information is recorded using the location of marks of a constant length, and mark edge recording (or PWM recording) whereby information is recorded using the length of the marks and the length of spaces between marks, are two methods of recording data to a phase change optical disk. The data recording density of mark edge recording is generally the higher of these two methods.
The mark edge recording method also generally records longer marks compared with the constant mark length in mark position recording. When a peak power laser beam is emitted to a phase change disk to record a long mark, heat accumulation in the recording film produces marks that are wider in the latter half of the mark as seen in the radial direction, something like a teardrop shape. Such marks significantly degrade signal quality, causing, for example, degraded signal linearity in the recorded signal, increased jitter during reproduction, mark remnants that are left when the marks are overwritten by direct overwrite recording, and signal crosstalk between tracks during reproduction.
Recording shorter marks and spaces is one means of increasing the recording density. A short space length, however, can result in thermal interference. For example, heat at the trailing end of a recorded mark is transferred through the following space, which can then contribute to a temperature increase at the beginning of the following mark. Heat at the beginning end of one recorded mark can also transfer through the preceding space and affect the cooling process at the end of the preceding mark. A problem with thermal interference in conventional recording methods is that mark edge positions will vary, causing a higher error rate during reproduction.
To address the above-noted problems, Japanese Unexamined Patent Application Publication (kokai) 7-129959 (U.S. Pat. Nos. 5,490,126 and 5,636,194) teach a method for recording marks by segmenting that part of the recording signal corresponding to a mark in mark edge recording into start, middle, and end parts, the start and end parts each having a constant pulse width and the middle containing pulses of a constant period. This recording signal is then used to rapidly switch the output of a two-value laser.
With this method, the width of the middle part of a long mark is substantially constant and does not spread because laser output is driven with a constant period pulse current producing the minimum power required for mark formation. An increase in jitter at the leading and trailing edges of the mark can also be suppressed during direct overwrite recording because the laser beam is emitted with a constant pulse width at the leading and trailing ends of the mark.
It is also possible to detect whether marks, or spaces before and after a mark, are long or short, and change the position at which the start and end parts of a mark are recorded according to the length of the mark and the leading and trailing spaces. This makes it possible to compensate during recording for peak shifts caused by thermal interference whereby heat at the end of a recorded mark transfers through the following space and affects the heating process at the beginning of the next mark, and heat at the beginning of a next recorded mark conversely travels back through the preceding space and affects the cooling process at the end of the preceding mark.
The publication Kokai 7-129959 does not, however, teach a method for determining the optimum positions of the start and end parts of a mark, nor does it teach a specific structure and basis for changing or adjusting the start and end edge positions.
If such an optimum method and structure are not defined, the reliability of optimized recording will be low. Furthermore, even if optimized recording is achieved, it will be at the expense of excessive time spent searching for the optimum position and excessive circuit cost.
A method for changing the start and end edge positions of a mark based on the data being recorded has been invented as a means of achieving high density data recording. A problem with this method, however, is that the edge of a recorded mark can move due to thermal interference as described above. This edge movement phenomenon is also highly dependent upon the disk structure and the composition of the recording film, and if either of these change even slightly, optimized recording cannot be achieved.